1. Field of the Invention
The present invention relates to a process for the preparation of N-alkylated anilines by reaction of anilines with lower alcohols or by reaction with anilines alkylated on the N-atom (transalkylation) in the presence of niobic acid or tantalic acid at elevated temperature.
N-Alkylated aromatic amines, which can be prepared according to the invention, are starting materials for the preparation of dyestuffs, pesticides, urethanes, pharmaceuticals, plant protection agents and the like. They are also used as mineral oil additives and as additives for surface coatings or other polymeric systems.
2. Description of the Related Art
Various processes for the preparation of secondary and tertiary aromatic amines are known. In general, they are prepared by alkylation of anilines with alcohols in the presence of acidic catalysts. Thus, it is already known, for the preparation of N-alkylated aromatic amines, to react aromatic amines with alkanols or the corresponding dialkyl ethers in the gas phase. For example, alumina or silicates are proposed as catalysts (for example silicates, such as Tonsil, in German Patent Specification No. 638,756). The said catalysts have the disadvantage that their activity decreases rapidly in the alkylation of aromatic amines (Houben-Weyl, Methoden der org. Chemie [Methods of organic chemistry], Volume XI/1 (1957) page 126).
It is also known from German Patent Specification No. 617,990 and DE-OS (German Published Specification) No. 2,335,906 that support materials which contain oxygen acids of phosphorus can be used for the said task. However, the service life and selectivity of such catalysts are not satisfactory for industrial processes.
Moreover, it was proposed in U.S. Pat. No. 2,580,284 to react aniline and methanol, using a catalyst which contains metallic copper on Al.sub.2 O.sub.3 and further oxides, such as zinc oxide, cadmium oxide, iron oxide, chromium oxide, calcium oxide, magnesium oxide or potassium oxide. Such catalysts suffer from the deposition of tarry substances even after a relatively short time. A further process for reacting aniline with methanol to give N-methylaniline according to DE-OS (German Published Specification) No. 2,061,709 is carried out on a chromium catalyst which can contain copper, zinc, iron, nickel or molybdenum as well as barium, magnesium or manganese. This process has the disadvantage that it is carried out under a pressure of 50-150 bar and is therefore too expensive for industrial application; this catalyst also does not reach a satisfactory service life.
According to DE-OS (German Published Specification) No. 2,120,641, copper chromite catalysts with barium, manganese, cerium and other elements as promoters are used for the preparation of secondary or tertiary aromatic amines from aliphatic alcohols and aromatic nitro compounds. However, this process leads to unsatisfactory yields.
Only a few and unsatisfactory processes are available for the preparation of anilines dialkylated on the N-atom, particularly for the preparation of N,N-diethylaniline.
In addition, it is in general desirable to be able, on the same catalyst, to set variable ratios of N-monoalkyl anilines to N,N-dialkyl anilines as required.
In U.S. Pat. No. 4,599,449, a process for the gas phase alkylation of aromatic amines with alcohols on transition metal oxides of subgroup 8 of the periodic table of the elements is described. However, the yield of alkyl anilines is very low. Even with a 5-fold excess of ethanol, the conversion of the aniline remains very low at a maximum of 38 %, and N,N-diethylaniline is obtained only in an inadequate yield. Furthermore, the proportion of ringalkylated by-products is very high in this process.
DE-OS (German Published Specification) No. 2,335,906 teaches the alkylation of arylamines with alcohols on silica catalysts which are coated with 0.1 to 20% by weight of phosphoric acid. To obtain good selectivities, however, a very large excess of alcohol of up to 20 mol equivalents is required. This has, on the one hand, the consequence of a low space-time yield and, on the other hand, the separation and recycle of the excess alcohol involves a high distillation cost. In order to avoid rapid deactivation of the catalyst and to ensure a long service life of the catalyst, it is necessary, furthermore, to feed phosphoric acid and/or phosphoric acid alkyl esters continuously during the alkylation; part of these phosphorus compounds is, however, always discharged and contaminates the reaction product, necessitating expensive separations.
It is also to use zeolites as catalysts for the gas phase alkylation of aromatic amines with alcohols. In U.S. Pat. No. 4,801,752, zeolites of the ZSM 5 type having an SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 20-700 are proposed. Flexible control of the yield in the direction of N,N-diethylaniline, is however, possible only to a certain extent. Under the most favourable conditions, a maximum molar selectivity of 10.1% for N,N-diethylaniline is achieved. Moreover, temperatures of up to more than 400.degree. C. must be applied in this process. In addition, a significantly large proportion of unidentified ring-alkylated by-products is formed.
U.S. Pat. No. 4,613,705 describes the alkylation of aromatic amines in the presence of a catalyst which is composed to the extent of at least 70% of a transition metal oxide from group V B of the periodic table of the elements and to the extent of up to 30% of tin (IV) oxide, for example V.sub.2 O.sub.5 /SnO.sub.2, Nb.sub.2 O.sub.5 /SnO.sub.2 and Ta.sub.2 O.sub.5 /SnO.sub.2. However, the conversion of aromatic amine is low, as is the selectivity for N,N-diethylaniline. By contrast, the proportion of undesired by products is very high. Therefore metal oxides of groups V B of the periodic table of the elements seemed to be unsuitable catalysts for the N-alkylation of anilines in general, and especially for the preparation of N-ethyl and N,N-diethyl anilines and in particular for the preparation of N-ethyl aniline.
It was therefore the object of the present invention to overcome the abovementioned disadvantages and to provide a technically feasible, more economical process for the preparation of N-alkylated anilines.